As a photodetector for a light spectrum detecting apparatus, a one- or two-dimensional photodetector utilizing a photodiode array or a CCD is used. Such a photodetector is put in a protective casing 24 so that a light receiving surface 23 can be protected from humidity or the like, as shown in FIG. 3. And a light transmitting plate 22 is provided in front of the light receiving surface 23 to seal an inert gas in the protective casing 24.
However, in some cases, a light incident to the light transmitting plate 22 is reflected by a back surface of the light transmitting plate 22 to return to a front surface thereof, and it is reflected again by the front surface and then incident on the light receiving surface 23 (such rays are shown with designation R in FIG. 3).
Such a multiply reflected light is a stray light, which causes to generate unnecessary signals when it enters the photodetector. As a result, the spectrum detection accuracy is lowered.
Further, it is difficult to remove such a stray light, because it is just adjacent to a light to be properly detected.